1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copier, a printer and facsimile device that uniformly charges the surface of a drum-shaped latent image carrier using charging means, develops the latent image formed on the surface of the charged latent image carrier to produce a visible image, and then finally forms this visible image on a recording material, and a process cartridge employed in same.
2. Description of the Related Art
The conventional charging means used in this type of image forming apparatus is a non-contact charging device such as a scorotron charger that charges the surface of a photosensitive drum that serves as a latent image carrier without contact therewith. However in order to generate discharge, non-contact charging devices generate the discharge products ozone and NOx which are known to create a variety of undesirable outcomes. For this reason, attention has switched in recent years to contact-type charging devices that generate a negligible amount of these discharge products.
In known contact-type charging devices the charging process is implemented by bringing of a contact charging member such as a charging roller or charging brush having a flat surface or a fixed-type or rotating-type charging brush into contact with a photosensitive drum surface. In these contact-type charging devices affixing material such as the toner affixed to the photosensitive drum surface is likely to affix to the contact charging member causing a drop in charging capacity over time. Of the contact charging members described above, from the viewpoint of the comparatively small drop in charging capacity that occurs over time, the rotating-type charging brush (brush roller) is the most useful. A known example of an apparatus that employs a roller of this type is described in Japanese Laid-Open Patent Application No. H11-84798. The image forming apparatus described in this gazette implements the charging process while a charging roller brush, serving as the brush roller, is rotationally driven in a counter direction to the surface movement direction of the photosensitive drum surface.
However, there is a drawback inherent to the implementation of a charging process based on the use of a brush roller such as this in that localized excessive discharge is likely to occur when, following contact with the photosensitive drum surface, the brush tip is separated from the photosensitive drum surface. The occurrence of this localized excessive discharge creates a problem of localized excessive discharge in a corresponding position on the photosensitive drum surface. This problem creates a condition whereby, as a result of the fact that, for example in so-called negative-positive developing, the electric potential of the areas of excessive discharge are unable to be lowered sufficiently even after exposure, the toner does not affix to these areas resulting in the generation of white spots on the image.
Technologies relating to the present invention are also disclosed in, for example, Japanese Patent No. 3,399,933, Japanese Laid-Open Patent Application No. S61-081666 and Japanese Laid-Open Patent Application No. H04-096080.